Automatic coffeemakers have a water reservoir and means for moving water from the reservoir and heating the water so that a small steady stream of water at correct extraction temperature is poured from above through the neck of a carafe or pot which holds the finished brew at an appropriate temperature.
Held interposed in that falling stream of hot water is a porous filter holding the ground coffee. The filter is held suspended by a filter holder, which generally holds the perimeter of the filter. The filter generally has a flat bottom and a fluted, upturned, and outwardly sloping fluted periphery. When poured into the filter, the ground coffee forms a thin layer along the flat bottom of the filter. As the narrow stream of hot water strikes the thin layer of ground coffee, it hits the center of the layer and passes through, thoroughly extracting the center portion. It may extract so thoroughly that it extracts bitter elements. Depending on the flow rate, thickness of the layer of ground coffee, and permeability of the filter, some of the hot water will flow laterally and extract the grounds that are around the center, but always at a lesser rate than at the center. Since the area of an annulus of a disc increases as the square of the distance from the center, it is apparent that much of the ground coffee on the filter will be unextracted, which is wasteful of an expensive commodity. In the prior art, distributors have been provided to break up the single water stream into multiple streams for more efficient extraction.